The present invention relates to an injection device comprising: a body receiving a product, having a needle and a piston attached to a piston rod movable between a storage position and an end-of-injection position, a sleeve for protecting the needle, movable between an initial position, where the needle is uncovered, and a final position, where the needle is covered, biasing means for moving said sleeve from its initial position to its final position, immobilization means for keeping said sleeve in its initial position, characterized in that: said piston rod comprises a first portion and a second portion, said second portion being able to switch between a locked position, in which said second portion is fixed to said first portion and an unlocked position, in which said second portion is movable with respect to said first portion on a predetermined distance along the longitudinal axis of said piston rod.
